THERE continues to be too much hype in the global-warming debate. Predictions of sea-level rise provide a case in point.
Thermal expansion of sea water and melting of land-based ice would drive the rise. The standard alarmist scenario then shows how that higher sea level would flood valuable coastal regions and drown some island nations.
The trouble with that kind of scenario is that no one can predict future sea levels reliably. They can't even know at this time how important man-made warming would be in determining the outcome.
William F. Tanner of the geology department of Florida State University at Tallahassee explained this recently in the Bulletin of the American Meteorological Society. He looked at sea-level changes for the past several thousand years as reflected by wave action on straight sandy beaches. The effects of the waves are "recorded" in the statistics of the distribution of sizes of quartz sand grains in old beach deposits.
He finds evidence for irregularly alternating high and low sea level with changes in the range of 1 to 3 meters. At least seven such events - four rises and three drops - occurred in the past 3,000 years, Dr. Tanner says. He estimates that the average duration for a high or low sea-level position was roughly five centuries.
The so-called "Little Ice Age" of the Middle Ages shows up well in his analysis. Sea level began to fall about AD 1200, reaching its lowest point in 2,000 years around AD 1400. This drop coincided with a cold climatic period. After that, Tanner says, sea level was rising by 1750 and has continued to rise until at least 1900. The nature of his data does not allow him to say much about this century. Tanner makes several important points:
* The steady sea-level rise since 1750 "apparently was not caused by human activity."
* His data can't show whether or not this natural warming is over. But "if global warming could be demonstrated ... this might very well be a continuation of the [natural] trend ... and therefore cannot be shown from available data to represent an effect caused by mankind."
* There is no such thing as an "absolute" normal sea level from which changes are measured. There are only the irregular few meter rises and falls which "appear to be inherent in the way the air-ocean-ice system operates at least under present conditions." It is against this background of natural fluctuations that any predictions of man-made sea-level changes have to be made.
The bottom line, Tanner says, is that one has to go back far longer than the instrumental record of the last century to build a reliable basis for projecting sea-level changes. He notes that data sets such as his "show clearly that other [than man-made] changes have been taking place." He adds that "therefore one cannot know in advance how the system will react during the interval of increased anthropomorphic influences: some other factor may be more important."
This is not an argument for or against the proposition that a man-made global warming is under way. It is not an argument for or against the possibility that such warming could bring an unnatural, perhaps dangerous, sea-level rise.
And it certainly is not an argument for or against the desirability of nations acting to curb the release of carbon dioxide and other heat-trapping gases.
But Tanner's study does make it clear that advocates of such action should not use supposedly science-based scary scenarios of drowned cities to bolster their case. Scientifically speaking, at this stage, they cannot know what they are talking about.